The present invention relates to three-phase AC motor drives and particularly to a filter device for power lines communicating between such drives and a connected motor.
Common AC induction motors use three-phase electrical power connected to stator windings of the motor. Each stator winding receives a different conductor of a three-phase power transmission line, where each conductor communicates an AC power waveform shifted with respect to the other conductors by plus and minus 120°.
Referring to FIG. 1, the motor 22 may connect directly to a power grid or, as shown, connect to a solid-state motor drive 10. In this latter case, the motor drive 10 will receive three-phase power 12 from the power grid at a rectifier 14 and the rectifier 14 will convert the received three-phase AC power into DC power on a DC link 16. The rectifier 14 uses an active or passive rectification system of a type known in the art.
The DC power on the DC link 16 is received by an inverter 18 which synthesizes new three-phase power 20, for example, having a different frequency than the three-phase power 12 to provide for motor speed control or control of other motor parameters.
The synthesized three-phase power 20 may be communicated to the motor 22 by means of a power cable 24. Such power cable 24 may extend for hundreds or even thousands of feet allowing the motor drive 10 to be located at a substantial distance from the motor 22. Such power cables 24 normally include three conductors (one for each power phase) and one or more ground conductors within a conductive shield, the latter to reduce the transmission of electrical interference to surrounding equipment.
The distributed inductance and capacitance of the power cable 24 can create electrical reflections along the power cable 24 resulting generally from an impedance mismatch between the characteristic impedance of the power cable 24 and the motor 22, at one end of the power cable 24, and between the characteristic impedance of the power cable 24 and the inverter 18, at the other end of the power cable 24. These reflections produce voltage and current surges 28 that can damage conductor insulation, cause arcing across motor bearings, and boost the voltage on the DC link 16 damaging the inverter 18 or causing the inverter 18 to shut down to avoid damage.
U.S. Pat. No. 5,990,654 assigned to the assignee of the present invention and hereby incorporated by reference describes a filter device 32 receiving the three-phase power 20 from the inverter 18 on one side through a relatively short cable where reflections are not a problem and attaching to the power cable 24 on the other side. The filter device 32 operates to reduce surges 28 by reducing reflections on the power line 24.
Referring now to FIG. 2, for this purpose, the filter device 32 may include a common mode choke 34 having three inductors 36, one attached to each conductor of each different phase of the synthesized three-phase power 20. Each of these inductors 36 is connected in series with one corresponding inductor 38 of three inductors 38 of a differential mode choke 40.
As is generally understood in the art, and as shown in FIG. 3, the common mode choke 34 forms each inductor 36 as a separate coil wound around a common core 46, with a well known geometry such as a toroidal core or a U-I core. In this way, the flux path passes through each inductor 36 in series so that each of the inductors 36 presents a relatively high impedance to common mode current components (those having a phase alignment) and a relatively low impedance to differential mode current components (those not aligned in phase).
In contrast, as shown in FIG. 4, the differential mode choke 40 forms each inductor 38 as a separate coil wound on one leg of an E-core 50 whose legs are bridged by an I-core 52 to provide a parallel rather than series flux path through each inductor 38. In contrast to the common mode choke 34, the differential mode choke 40 presents a relatively high impedance to differential mode current components and a relatively low impedance to common mode current components.
The common mode choke 34 and differential mode choke 38 can also be wound on a single magnetic structure 41, as shown in FIG. 5. The differential mode portion of this choke is formed by each inductor 38 as a separate coil wound on one leg of an E-core 50 whose legs are bridged by an I-core 52. The common mode portion of this choke is formed by each inductor 36 as a separate coil wound around the common U-core 46 bridged by the I-core 52, which is shared with the differential mode portion of the choke. Each of the common mode inductors 36 is connected in series with one corresponding differential mode inductor 38. This series combination provides high impedance to both common mode and differential mode current components.
Referring still to FIG. 2, each of the inductors 38 of the differential mode choke 40 is shunted by a resistor 42 connected in parallel with the individual inductor 38. These resistors 42 have impedance selected to provide a matching to a characteristic impedance of the power cable 24 to thereby reduce reflections on the power cable 24 at the interface between the filter device 32 and the power cable 24.
While the filter device 32 may significantly reduce reflections on the power line 24, significant surges 28 may still occur in certain cases.